1. Field of the Invention
The present invention relates to a method for recovering zinc oxide from dust containing iron oxide and zinc oxide (ZnO), which dust is generated from steelworks, and also relates to an apparatus therefor.
2. Description of the Related Arts Steelworks which produces iron from raw materials such as iron ores and scrap generates dusts containing large amount of iron oxide. The generated dusts are hereinafter referred to as "steelwork dusts". Typical steelwork dusts are blast furnace dust and converter dust. Percentage of sum of blast furnace dust and converter dust in the total dust generated from steelworks reaches as high as 60%. Main components of blast furnace dust are carbon and iron oxide, and main component of converter dust is iron oxide. Both of these dusts contain zinc ranging from 0.1 to 3 wt. % in a form of mainly zinc oxide, though a small amount of metallic zinc exists.
For recovering iron and carbon components from steelwork dust, steelworks recycles the dust. Since, however, zinc induces scaffolds on the wall within blast furnace, strict control of zinc quantity accumulated in the blast furnace is required for recycling the dust as a raw material to charge into the blast furnace while securing stable operation thereof. Current practice is to limit the quantity of zinc being charged into the blast furnace to 0.1 kg or less per ton of pig iron.
To do this, steelworks separates the dust into high concentration zinc dust and low concentration zinc dust by a dezincing unit such as wet cyclone, and recycles only the low concentration zinc dust as a raw material for charging into the blast furnace. High concentration zinc dust is treated by landfilling. Recently, however, the discharged quantity of high concentration zinc dust has increased to induce a problem of difficulty in dust treatment.
On the other hand, a large quantity of scrap are accumulated in Japan. As a result, the percentage of scrap in available iron source has been increasing even in an integrated process steelworks from the standpoint of effective use of resources. In addition, the percentage of galvanized steel sheets in scrap has increased, which also increases the zinc content in the raw material in steelworks.
Responding to the recent tendency of increasing zinc content in raw materials for steelworks as described above, technology of treating high concentration zinc dust not only by landfilling but also by positively recovering zinc as a zinc source in smelting process is under study. For utilizing the dust as zinc source in smelting process, the zinc concentration is required to be increased as high as possible. In this respect, current target level of the zinc concentration is set to 50 wt. %.
Other than above-described blast furnace steelworks, what is called the electric furnace steelworks also discharge dust. The electric furnace dust contains, however, mainly iron oxide and zinc oxide, accounting for 20 to 30% of iron oxide and 15 to 25% of zinc oxide. Although about 60% of the generated electric furnace dust are recycled as a zinc source for smelting, remained 40% of them are discarded to landfill. Consequently, electric furnace dust also waites for extended use field as a zinc source for smelting as in the case of blast furnace dust and converter dust.
Consequently, for recovering zinc from the above-described steelwork dust and the like to use as the zinc source for smelting process, an important issue is how to concentrate zinc to a level of 50% or more in an efficient way.
Unexamined Japanese patent publication No. 6-145830 discloses a method for recovering zinc from steelwork dust. The technology is described below referring to a drawing. FIG. 4 illustrates the process flow of the technology.
An steelwork dust 2 which was stored in a first hopper 1A is sent to a mixer 4 along with a binder which was stored in a binder hopper 3, then the dust and the binder are mixed together in the mixer 4. The mixture of dust 2 and binder is charged to a pan pelletizer, where the mixture is pelletized. The pellets are cured in a rapid curing unit 6, then they are dried in a drier 7.
The pellets 10 are stored in a pellet hopper 8, from which they are charged into a molten iron ladle 9 along with a molten iron 11. The pellets 10 charged simultaneously with the molten iron 11 are entangled in the molten iron 1, thus dispersing the pellets into the molten iron 11. The iron oxide in the steelwork dust 2 reacts with carbon and silicon in the molten iron and is reduced to iron and recovered in a form of iron into the molten iron.
The zinc oxide in dust is reduced by carbon in the molten iron and vaporized. Then the vaporized reduced zinc oxide is oxidized in air to form fine zinc oxide particles having a size of 10 .mu.m or less. Above the surface of molten iron, vaporized iron and degradated cold pellets float, which are then collected by a dust collector 13 along with the particles of zinc oxide via a dust collection duct 12.
For increasing the zinc concentration in dust which is collected by the dust collector 13, the dust collected by the dust collector 13 is sent to a second hopper 1B again via a distributor 15 to store, followed by a similar processing as described above. The treatment increases the zinc concentration in dust, and finally the dust recovered in a hopper 1E has 50 wt. % or more zinc concentration, which concentration is significantly high compared with the zinc concentration of 0.1 to 3 wt. % in the original steelwork dust.
The above-described technology, however, has the following problems. The number of dust collector 13 is only one. The dust containing zinc, which is collected by the dust collector 13, is pelletized again for all of them. The re-pelletized dust is charged into the molten iron ladle. The procedure is repeated for 2 to 3 times until the zinc concentration in the dust reaches a target value.
Analysis of thus treated dust revealed that, if the average zinc concentration of the total dust failed to reach the target value, the whole dust have to be re-pelletized and re-reduced in the molten iron ladle, even if a part of the dust were concentrated to a high level. Accordingly, the treatment cost and time for pelletizing, handling, and reducing increase. In addition, increase in the amount of dust for re-treatment increases the investment cost for zinc recovery.
Since the treatment apparatus processes the dust for several times, the necessary scale of apparatud becomes large for the quantity of steelwork dust being processed. Furthermore, the apparatus stops operation thereof when the treated dust reaches a specified concentration, and resumes operation with a fresh raw material. That is, the apparatus is operated in batchwise mode, and the efficiency is unfavorable.